Shoe soles and midsoles are provided with a variety of different properties that may, depending on the specific type of shoe, be realized to different extents. Some of the earliest midsoles were a thin rubber layer, but this material did not provide much in the way of comfort to the wearer.
To increase wearer comfort, soles and midsoles have been increasingly formed of material with voids formed therein to increase the cushioning feel, commonly referred to as foam materials. The foam soles and midsoles absorb impact and protect the wearer's feet during running. These materials compress on impact and expand back to its original shape in order to compress again before the next foot strike. These foam soles and midsoles are formed from polymer pellets containing a blowing agent (foamable or expandable pellets) that are then foamed when they enter the mold to form a foam midsole.
Foam soles and midsoles are generally made of foam plastic polymers, such as ethyl vinyl-acetate (“EVA”) or polyurethane (“PU.”) PU is usually denser and heavier than EVA with better durability, but PU midsoles usually lack the same energy of EVA midsoles. The EVA and PU midsole technology has changed in recent years with respect to weight, performance, and cosmetic geometry. There is a need for new foams that satisfies these demands.
Recently, shoe soles and midsoles have been produced using a newly developed material called expanded thermoplastic polyurethane (“eTPU”). The thermoplastic properties of the material allow the material to be formed into tiny beads, each bead enclosing tiny pockets of gas, with the beads bonded together to form a granular but closed plastic foam structure. WO 2005/066250 describes a shoe sole formed out of eTPU, but the properties of the sole are influenced by the eTPU continuously over the entire surface such that fine control over the sole properties is not possible. U.S. Publication No. 2013/0291409 describes a midsole formed of eTPU that incorporates a second region that is free of eTPU and is formed of a material that is stiffer than eTPU such that the second region is able to increase the stability of the eTPU to provide fine control over the sole properties.
The shoe soles and midsoles formed of eTPU have satisfied the need for a new and improved midsole material, but there is still a continuing drive for new foams that continue to reduce weight without impacting performance while providing a new look and feel. The present invention is lighter weight than eTPU and EVA and is a longer lasting foam. The material is also greener than EVA, and requires less processing energy, which improves efficiency and energy consumption.